The official guidelines for ICD-10-CM define the conventions used in the code set. In section 1.A.6, we learn the following about some common abbreviations in the Tabular List:

NEC “Not elsewhere classifiable.” This abbreviation in the Tabular List represents “other specified”. When a specific code is not available for a condition the Tabular List includes an NEC entry under a code to identify the code as the “other specified” code.

NOS “Not otherwise specified.” This abbreviation is the equivalent of unspecified.

Section 1.A.9 tells us a little more:

Codes titled “other” or “other specified” are for use when the information in the medical record provides detail for which a specific code does not exist. Alphabetic Index entries with NEC in the line designate “other” codes in the Tabular List. These Alphabetic Index entries represent
specific disease entities for which no specific code exists so the term is included within an “other” code.

Codes titled “unspecified” are for use when the information in the medical record is insufficient to assign a more specific code. For those categories for which an unspecified code is not provided, the “other specified” code may represent both other and unspecified.

For example, suppose we report the following code:

M50.81 Other cervical disc disorders, high cervical region

This would be appropriate if the documentation suggests some sort of disc disorder, but the other options in the code set do not match. The other choices include disc disorder with myelopathy and
radiculopathy, displacement, and degeneration. If the patient has one of these other things, we should look to those codes.

Suppose we document: “the patient suffers from spinal radiculopathy”. The correct code would be:

M54.10 Radiculopathy, site unspecified

If we had documented where the radiculopathy occurs, we could find a more specific code.

Suppose we specifically document: “the patient has facet syndrome at L3/L4”. There are no codes with the description “facet syndrome”. But, we documented a specific condition. Therefore we are looking for an “other” or “NEC” code that works. This might be the one:

M53.86 Other specified dorsopathies, lumbar region

This is the proper application of “other” in ICD-10. This code could also work for any specific lumbar dorsopathies that have no code.

The bottom line is that you should select the code that most accurately reflects what was documented. Don't avoid certain codes just because you don't understand them. Rather, learn the conventions of the code set and get it to work with you.